Catheters have been inserted into humans and animals or monitoring and treatment purposes and such catheters have to be small in size and flexible in structure in order to function without irritating the body part into which they are placed. Typically, catheters are used to infuse medications or remove samples for purposes of analysis. Multilumen catheters are sometimes used to infuse medication and remove samples at the same time.
If a sample is removed for purposes of analysis, it has to be taken to a laboratory, where the analysis is made so results can be transmitted to the doctor. Delay in performing the analysis and transmitting results sometimes can be fatal to the patient. A use for a catheter is for access to the vasculature for a hydraulic column that transmits pressure readings to an external pressure sensor. Hydraulic columns have problems of air bubbles, kinks in the column tubing and blood clots, each of which tend to affect the reliability, waveform fidelity and the accuracy and precision of the readings.
Current technology uses a 20 gauge catheter to introduce therapy or provide diagnosis. This size is easily inserted and used without irritation or injury to the body. Twenty gauge catheters are commonly used on all but pediatric patients in the peripheral vessels without problems of introduction or irritation. A pressure sensor associated with a 20 gauge catheter provides an easy placement but does not eliminate hydraulic column difficulties. Typical of cumbersome external pressure sensors is the unit shown in U.S. Pat. No. 4,825,876 which has a flow through passage to which a sensor is connected in fluid communication. External pressure sensors are typical separated from the patient to prevent damage or dislodgement when the patient moves. The size of such external pressure sensors are typically two or three times a standard female luer.
Catheters having sensors are known and include sensors mounted at the distal tip of the catheter. U.S. Pat. No. 3,710,781 shows a catheter tip pressure sensor wherein a pair of elongate pressure sensor elements are mounted on opposite sides of a support. This is done to permit as large a sensor area as practical for purposes of providing accurate reproductions of blood pressure waveforms. U.S. Pat. No. 3,545,275 shows a device responsive to impedance used for measuring pressure with a miniaturized sensor. The sensor is responsive to diaphragm fluctuations in the distal part of a small diameter tube.
A small probe is disclosed in U.S. Pat. No. 3,811,427 wherein a pair of electrodes are mounted in a liquid filled chamber and are sensitive to fluctuations in a diaphragm mounted at the distal part of a catheter tube. The probe is said to be smaller than one millimeter. Two embodiments are shown. Each has a diaphragm in the distal part of the catheter and a longitudinal separator which carries the pressure responsive means and isolates the liquid from the remainder of the catheter such that fluctuations in the diaphragm are transmitted to the separator which is generally longitudinally disposed.
U.S. Pat. No. 4,722,348 shows a semiconductor mounted within a tubular housing in the part of the catheter tube and having a pressure inlet. Sealant protects the semiconductor which is held to the support by the double face adhesive tape which also carries the electrical conductors. U.S. Pat. No. 4,809,704 discloses catheters with the sensor mounted in the tip of the catheter supported on a base by a potting resin carried within the catheter tube. The resin is a urethane or silicone material about the sensor with appropriate openings for sampling. Assembly of the sensors within the catheters has been slow and labor intensive and the wiring to the sensor is difficult and subject to serious signal and noise.
The offset pressure due to changes in atmospheric pressure has to be accounted for in that the reference side of the sensor is considered a zero pressure. U.S. Pat. No. 4,672,974 has an apparatus with a port for a substitute reference pressure and an external pressure gauge for measuring the mean pressure through an auxiliary lumen of the catheter. Thus, a known pressure can be substituted for atmospheric pressure in the process of rezeroing the offset pressure. U.S. Pat. No. 4,712,566 has a sensor carried on a guide which is moveable in and out of the catheter tube so that the in vivo side of the sensor may be brought into the catheter removed from blood pressure and subjected to a generated pressure during calibration. The guide and catheter cooperate with one another to seal off the sensor during rezeroing.
U.S. Pat. No. 4,854,326 has an impedance variable transducer with a technique for zeroing the in vivo transducer by varying the static pressure in a reservoir connected to the transducer. Thus, changes in the height of the reservoir can be used to adjust the zero point of the transducer. This approach, although feasible, introduces another variable into the system. In addition, a gas retaining flexible membrane has to be located in the liquid filled lumen to the reservoir. The membrane is to separate the liquid from the gas filled lumen. The flexible membrane is said to prevent oscillating movement of the liquid in the lumen as a result of the interchange of energy by the liquid, the displacement of the diaphragm in the transducer and the compliance of the lumen about the liquid. Isolation is not the purpose of the flexible membrane.
U.S. Pat. No. 3,565,056 shows a catheter for placement within a body and an external part on the catheter connected to a domed chamber substantially filled with a strain gauge sensor. A saline solution is supplied to fill the chamber and the catheter during pressure measurement via a conduit passing through parts of the chamber. The conduit passes through of a wall of the chamber to fill the chamber and the catheter at a slow rate with neglible back pressure.
A large part of the problem with pressure transducers is to provide a hydraulic column between the body and the sensor. That is to say that structure should allow prompt and thorough debubbling of the catheter lumen to the body and the associated passage to the sensor. The '056 patent discloses plumbing for a sensor chamber which allows the slowly filled chamber and catheter to block flow of bodily fluid into the catheter. External sampling and means for permitting air within the system to escape are not disclosed. The formation of a solid hydraulic column between the body and the sensor can not be quickly and easily accomplished.
The '056 teaches the profusion of saline solution at a low rate to control filling of the domed chamber. Pressure changes within the domed chamber are claimed to be neglible. An extremely small passage is provided in order to choke the flow entering the dome chamber. It is contemplated that the catheter be flushed prior to insertion into the vein or artery in order to prevent or eliminate air embolism. Consequently, the disclosure of '056 does not suggest or teach the idea of connecting to a standard connector on the proximal part of an inserted over the needle catheter and adapter. The '056 does not teach how a passage extending from the distal part to the proximal part of a connector facilitates forming a hydraulic column. In addition, there is no teaching of the method of removing the connector and sensor from the catheter adapter in order to have atmospheric pressure on both sides of a pressure transducer for purposes of rezeroing.